Question

1. Given that a recrystallization involving co-solvents requires that the two solvents be miscible but have differing polarities, which solvent pair below would be ideal for a recrystallization?

H2O and CH3OH

CH3OH and C6H14

H2O and CH3OCH3

2. The solubility of Compound A is shown below in methanol, water, ethyl acetate and toluene. Given that all of the other requirements for a recrystallization are met, which solvent would be the best for the recrystallization of 3 grams of solid A?

Water. Solubility is 8 g per 100 mL when hot and 0.2 g per 100 mL when cold

Toluene. Solubility is 2 g per 100 mL when hot and 25 g per 100 mL when cold

Ethyl acetate. Solubility is 0.2 g per 100 mL when hot and 0.1 g per 100 mL when cold

None of the solvent choices would work

Methanol. Solubility is 58 g per 100 mL when hot and 30 g per 100 mL when cold

Answer 1

1) Methanol is very much polar while hexane is non-polar so the two solvents are not miscible.
On the other hand the rest of the two options are miscible and polar but CH₃OH has
-CH₃ part which is not polar (even though the presence of -OH makes it polar) so there is
high chance of giving crystals. CH₃OCH₃ is polar and forms hydrogen bonding but the polarity
difference is huge
So the answer should be H₂O and CH₃OH

2) For toluene we can observe that solubility decreases with increase in temperature(when hot)
which reverses the reaction rate and aiding the crystallization process on a whole.
   For other options the phenomena is quite opposite. So, the answer is toluene.